This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
With the evolution of Long Term Evolution (LTE) system, LTE network switches from a homogeneous network into a heterogeneous network, where Macro eNode-Bs (eNBs) have higher transmission power for coverage purpose and pico eNBs have lower transmission power for capacity purpose. As verified, the handover failure rate is increased in such a heterogeneous network. It is therefore proposed that UE is connected to both a Macro eNB and a pico eNB concurrently, which is called dual connectivity, as shown in FIG. 1.
Dual connectivity is a feature defined from the UE's perspective, where a UE may simultaneously receive from and transmit to at least two different network points as shown in FIG. 1. Dual connectivity is one of the features that are being standardized within the umbrella work of small cell enhancements within 3GPP Rel-12.
Dual connectivity is defined for the case when the aggregated network points operate on the same or separate frequency. Each network point that the UE is aggregating may define a stand-alone cell or it may not define a stand-alone cell. It is further foreseen that from the UE's perspective, the UE may apply some form of Time Division Multiplexing (TDM) scheme between the different network points that the UE is aggregating. This implies that the communication on the physical layer to and from the different aggregated network points may not be truly simultaneous.
Dual connectivity as a feature bears many similarities with carrier aggregation and Coordinated Multi Point Transmission/Reception (CoMP). The main differentiating factor is that dual connectivity is designed considering a relaxed backhaul and less stringent requirements on synchronization requirements between the network points. This is in contrast to carrier aggregation and CoMP, where tight synchronization and a low-delay backhaul are assumed between connected network points.
Due to complicacy, some UEs support dual connectivity at Layer 2 and Layer 3. In other word, their physical layer can only connect with either Macro eNB or pico eNB at the same time slot. In order for this type of UE to work in dual connectivity scenario, subframes have to be split into two sets, subframes within one set are used for communications between UE and Macro eNB, subframes within the other set are used for communications between UE and pico eNB.
Then one issue is how to guarantee the second random access procedure success for this TDM type UE in dual connectivity scenario, as there is strict timing requirement for MSG3 transmission in contention based random access or MSG1 transmission in contention free random access procedure according to the specification (referring to Section 8 in the 3GPP Technical Specification 3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, v.11.1.0).